Lecture 15 - Cytoskeleton & CM: Microtubules

Question 1

what are the 5 roles of the cytoskeleton?

Answer 1

1. scaffolding that provides structural support
2. network of tracks to direct the movement of material
3. force generating apparatus for movement and contraction
4. a framework for positioning various organelles within the cell
5. essential component of the cell division machinery

Question 2

__________ are the largest of the cytoskeletal components of the cell

Answer 2

microtubules

Question 3

what are the 2 types of microtubules in the cell?

Answer 3

1. cytoplasmic microtubules
2. axonemal microtubules

Question 4

what are cytoplasmic microtubules responsible for?

Answer 4

• maintaining axons
• formation of mitotic and meiotic spindles
• placement and movement of vesicles
• maintaining or altering cell shape

Question 5

what are axonemal microtubules responsible for?

Answer 5

• cilia
  -flagella
• basal bodies to which cilia and flagella attach

Question 6

def: the central shaft of a cilium or flagellum, is a highly ordered bundle of MT’s

Answer 6

axoneme

Question 7

what are the protein building blocks of microtubules?

Answer 7

tubulin heterodimers

Question 8

MT’s usually consist of 13 longitudinal arrays of polymers called ___________

Answer 8

protofilaments

Question 9

what is the basic subunit of a protofilament?

Answer 9

heterodimer of tubulin, one alpha-tubulin and one beta-tubulin
bind covalently to form alpha-beta- heterodimer

Question 10

T or F: all the dimers in the MT are oriented the same way

Answer 10

true

Question 11

as a result of their dimer orientation, all protofilaments have _______ _________

Answer 11

inherent polarity

Question 12

cytoplasmic MTs are _____ ______ with 13 protofilaments?

Answer 12

simple tubes

Question 13

some axonemal MTs form _______ or ________ MTs

Answer 13

doublet or triplet

Question 14

where can doublet MTs be found

Answer 14

in cilia and flagella

Question 15

where can triplet MTs be found

Answer 15

in basal bodies and centrioles

Question 16

when does reversible polymerization of tubulin dimers occur?

Answer 16

only in the presence of GTP and Mg2+

Question 17

def: dimers aggregate into oligomers, which serve as “nuclei” from which new MTs grow

Answer 17

nucleation

Question 18

def: the addition of more subunits at either end

Answer 18

elongation

Question 19

how do the lag phase and elongation phase compare

Answer 19

the lag phase starts off slower since nucleation is a slow process but then the elongation phase is much faster

Question 20

def: when the concentration of tubulin becomes limiting, the assembly is balanced by disassembly

Answer 20

plateau phase

Question 21

def: concentration in which MT assembly is exactly balanced by disassembly

Answer 21

critical concentration

Question 22

where does the addition of tubulin occur quicker?

Answer 22

the plus ends of the microtubules

Question 23

T or F: the plus and minus ends of microtubules have the same critical concentation

Answer 23

false, they differ

Question 24

def: addition of subunits at the plus end, and removal from the minus end

Answer 24

treadmilling

Question 25

if [tubulin subunits] is above critical concentration for the plus end but below the minus end, _________ will occur

Answer 25

treadmilling

Question 26

when is the GTP bound to the submit hydrolyzed to GDP?

Answer 26

after the heterodimer is added to the MT

Question 27

is GTP hydrolysis required for MT assembly?

Answer 27

no, it is needed to promote interactions and addition to MTs

Question 28

def: one population of MTs grows by polymerization at the plus ends whereas another population shrinks by de-polymerization

Answer 28

dynamic instability model

Question 29

the GTP cap at the plus end prevents _________ __________

Answer 29

subunit removal

Question 30

growing MTs have _____ at the plus ends, and shrinking MTs have ______

Answer 30

GTP, GDP

Question 31

hydrolysis of GTP after assembly changes the conformation of the subunits and tends to force the protofilament into.......

Answer 31

a curved shape that is less able to pack into the microtubule wall

Question 32

which end of the microtubule has a higher critical concentration and therefore will grow slower?

Answer 32

the negative end, the plus end will grow faster at the same concentrations

Question 33

def: where MTs originate from, serves as a site of MT assembly nucleation and acts as an anchor for one end(minus)

Answer 33

microtubule-organizing centre (MTOC)

Question 34

an example of an MTOC in cells during interphase is

Answer 34

centrosome near the nucleus

Question 35

centriole walls are formed by ___ ____ ___ ________ and are oriented at ___ ____ to each other

Answer 35

9 sets of triplet microtubules, right angles

Question 36

can cells divide without centrioles?

Answer 36

yes, but they have poorly organized mitotic spindles

Question 37

where are gamma-tubulins found ?

Answer 37

only in centrosomes

Question 38

def: nucleate the assembly of new MTs away from the centrosome

Answer 38

gamma- tubulin ring complexes (gamma-TuRCs)

Question 39

what does loss of gamma-TuRCs prevent?

Answer 39

prevents the cell from nucleating MTs

Question 40

what is the most important role of MTOCs?

Answer 40

the ability to nucleate and anchor MTs

Question 41

which end of the MTs are anchored in the MTOC

Answer 41

the minus end

Question 42

what are the 4 ways that cells control microtubule assembly and structure?

Answer 42

1. MT-binding proteins that use ATP to drive vesicle or organelle transport or generate sliding forces between MTs 2. Microtubule-Stabilizing/Bundling Proteins 3. Plus-End Tubulin Interacting Proteins 4. Microtubule-Destabilizing/Severing Proteins

Question 43

def: bind at regular intervals along a microtubule wall, allowing for interaction with other cellular structures and filaments

Answer 43

Microtubule-Stabiliziing/Bundling proteins

Question 44

def: MAP that causes MTs to form tight bundles in axons

Answer 44

Tau

Question 45

def: MAP that promotes the formation of looser bundles in dendrites

Answer 45

MAP2

Question 46

what part of the MAP controls the spacing of MTs in the bundle?

Answer 46

the extended arm

Question 47

can MTs stay intact for long periods of time?

Answer 47

no, they are too unstable, and will de-polymerize unless they are stabilized in some way

Question 48

what can stabilize MTs?

Answer 48

+-TIPs capture the growing plus end and protect it form catastrophic subunit loss

Question 49

def: bind to tubulin heterodimers and prevent their polymerization

Answer 49

Stathmin/Op18

Question 50

def: act as the ends of microtubules, promoting the peeling of subunits from the ends

Answer 50

catastrophins

Question 51

def: proteins that sever MTs

Answer 51

katanins